1. Field of the Invention
The present invention relates to an apparatus and method for applying traction to an elongate element which is produced by fusing a preform of glass material and is usable in a process for producing an optical fibre.
2. Description of the Related Art
As is known, an optical fibre is produced by a process of drawing a preform of glass material. In particular, there is a known method of placing the preform in a vertical position inside a furnace to cause the fusion of a lower portion of the preform. The fused material is then stretched downwards by a traction device, thus producing a threadlike element which forms the optical fibre.
The patent EP 367871 in the name of Corning Glass Works describes a method for making an optical fibre of the step index type, having a sharp change in the refractive index between the core and the cladding. This method comprises an initial stage of depositing particles of glass comprising a base glass and a refractive index-increasing dopant on a support (“mandrel”). The mandrel is then removed and the resulting preform (“soot preform”) is consolidated in such a way as to form a core preform having a surface region with a low dopant content. The core preform is stretched and the hole present in it is closed in such a way as to form a core bait rod. The cladding glass soot is then deposited on the core rod at a density of at least 0.5 g/cc. This is done by directing the flame of an auxiliary burner onto the core bar immediately before the cladding soot is deposited on it. In this way, a final preform is produced, and this is consolidated and drawn to produce an optical fibre.
A process of the type described above for producing a final preform is commonly known as the OVD (outside vapour deposition) process.
Also according to the patent EP 367871, the core preform is stretched by using a traction device comprising a pair of powered traction wheels (indicated by 52 in FIG. 3 of the patent in question) which apply a downward traction on opposite sides of the core rod.
The applicant has noted that, in the execution of this last stage, it is possible that, as a result of an imprecise positioning and/or operation of the traction wheels, opposite sides of the core rod undergo different stresses which cause the core rods to bend during its forming process; therefore the production of core rods which are not perfectly rectilinear, in other words which have shape defects, is possible.
The applicant has also noted that the bending of the core rod may also be introduced by non-uniformities of the temperature within the vertical furnace.
GB 1315447 relates to an apparatus adapted for drawing pipes or bars, particularly quartz ones, from blanks of large diameter, and addresses the problem of sagging of the pipe in a heating zone of a known apparatus, which causes a general deformation of the finished quartz pipe, as well as an inadequate fusion of the pipe. The apparatus of GB 1315447 comprises a head adapted for feeding a blank to a system of gas burners, a head for drawing a finished pipe, a drive with a shaft servicing both heads and providing for rotational movement of the pipe and the blank, and two further drives servicing both heads and providing for axial movement of the pipe.
The applicant has observed that the aforesaid shape defects may lead to the presence of a non-negligible error of straightness of the core rod during the final stage of chemical deposition.
The applicant has therefore noted that these shape defects may lead to the production of a final preform (from which the optical fibre will subsequently be drawn) which is non-uniform, in other words a preform whose central portion (formed by the core rod) is curved and is at a non-constant radial distance from the outer surfaces of the preform; in other words, if a cross section of the preform is viewed, it may be seen that the central circular portion corresponding to the section of the core rod is not concentric with the circular section corresponding to the section of the final preform. This error of concentricity is maintained during the drawing stage, and the optical fibre which is produced therefore has a core which is not concentric with the cladding.
The core/clad concentricity is a basic parameter of an optical fibre, since it provides a measurement of the extent to which the axis of the core is aligned with the axis of the cladding. More particularly, the concentricity is defined as the distance between the axes of the core and the cladding. Typically, the concentricity must be small (for example, less than 0.5 μm and preferably less than 0.3 μm), so that, when two end portions of two different optical fibres are joined, the attenuation of the transmitted light is low. This is because optical fibres are typically joined by aligning the outer surfaces of their claddings, and therefore, if the cores are not perfectly positioned along the axes of the corresponding fibres, the joint between the two cores may be partial, producing a high-loss joint.